This invention relates to a method to trigger the power-conducting state of a gate-turn-off, GTO, thyristor. The invention also relates to an apparatus to perform this method.
Thyristors which can be turned off by a suitable control current connected to their gate terminals permit a significantly simpler design of converters, direct d.c. converters and other equipment used in power electronics than traditional thyristors, which can only be switched on by an ignition current and thereafter retain their power-conducting state until the anode-to-cathode current flow reverts to zero, either by natural means or by the introduction of a counter-voltage so that they are extinguished automatically.
The conducting state of a GTO thyristor is thus initiated by a positive ignition current pulse on the gate. There is a danger in using a GTO thyristor due to the internal structure particularly in case of a low load condition (a low current between anode and cathode). The thyristors partially extinguish, i.e., in one portion of their current-conducting cross-section, and even with a renewed increase in current do not revert to their full conducting state. Such an incomplete conducting state can lead to a breakdown of the GTO. It could even lead to the situation that the thyristor extinguishes at a low current level so that a current block results despite a desired conducting state. It is thus necessary, even after an initial ignition pulse with a GTO thyristor, to trigger this thyristor using an ignition current in accordance with the current-conducting state to avoid any partial or total extinction in case of a low current load.
For that purpose either a constant control current or a preset sequence of ignition current pulses could be transmitted through the GTO during the entire current-conducting period. This, however, would involve very high energy requirements which during the switching cycle could often represent the greatest portion of the total control energy.
The object of this invention is to ensure a reliable current-conducting state of said GTO. A second object of this invention is to provide said current-conducting state using reduced control power into the thyristor gate circuit.